Re: [PATCH] net: linearizing skb when downgrade gso_size

Herbert Xu wrote:

On Tue, Jul 09, 2024 at 11:53:21AM -0400, Willem de Bruijn wrote:

quoted

quoted

+		/* Due to header grow, MSS needs to be downgraded.
+		 * There is BUG_ON When segment the frag_list with
+		 * head_frag true so linearize skb after downgrade
+		 * the MSS.
+		 */

This sounds completely wrong.  You should never grow the TCP header
by changing gso_size.  What is the usage-scenario for this?

Think about it, if a router forwards a TCP packet, and ends up
growing its TCP header and then splits the packet into two, then
this router is brain-dead.

This is an unfortunate feature, but already exists.

It decreases gso_size to account for tunnel headers.

For USO, we added BPF_F_ADJ_ROOM_FIXED_GSO to avoid this in better,
newer users.